As described in, for example, Patent Document 1 (Japanese Patent Application Publication No. H11-341576), a gooseneck condenser microphone is preferably used in conference facilities such as international conference rooms from the viewpoint of its simple appearance and ease of adjustment of the angle and height of the microphone.
In the gooseneck condenser microphone, a condenser microphone unit and an output module section are formed separately. The condenser microphone unit is supported on the output module section via a support pipe including a flexible pipe in whole or in part, so that the condenser microphone unit can easily be brought close to the mouth of a speaker by the flexibility of the flexible pipe.
Usually, the condenser microphone unit incorporates a field effect transistor (FET) serving as an impedance converter, and the output module section is provided with a circuit board having a sound signal output circuit and a power supply transformer in a cylindrical shield case. The condenser microphone unit is connected to the circuit board in the output module section via a dedicated microphone cable inserted through the support pipe.
The microphone cable includes a power wire for supplying power to the condenser microphone unit, a signal wire for giving sound signals, which are generated from the impedance converter, to the output module section, and a shielding covered wire that electrostatically shields the power wire and the signal wire and connects them to the ground. As the shielding covered wire, a two-core shielding covered wire the outer peripheral surface of which is covered with an external sheath (skin) is used. The output module section is sometimes called a power module section because it supplies power to the condenser microphone unit.
The microphone cable is vulnerable to noise (electromagnetic waves) coming from the outside because the sound signals are transmitted unbalancedly. Therefore, if strong electromagnetic waves are applied to the microphone cable, the electromagnetic waves intrude into the condenser microphone unit and the output module section, being detected by a semiconductor device etc., and therefore noise is sometimes generated.
In particular, a cellular phone that has come into wide use rapidly in recent years generates considerably strong electromagnetic waves (for example, within the range of about several centimeters to several tens centimeters, a field intensity reaching tens of thousands times of field intensity produced in the city by commercial electric waves). Therefore, in the field of microphone, measures against electromagnetic waves caused by cellular phones are urgently needed.
The flexible pipe functions as a shield for the microphone cable, but this function is not complete. Specifically, the flexible pipe is manufactured by forming a coil spring, which exerts a restoring force, by a round wire rod formed of steel etc. and by inserting a triangular wire rod formed of a copper alloy, which has a triangular cross-sectional shape and is plastically deformed, into the gap of the coil springs from above, so that the contact portion of these wire rods has impedance though the resistance value is low (for example, about 1Ω).
The condenser microphone unit and the output module section each have a sure shield case. However, when viewed as the entire of microphone, the shield in the portion of the flexible pipe is incomplete. Therefore, strong electromagnetic waves intrude into the microphone through the flexible pipe, and resultantly noise may be generated as described above.
To solve this problem, the applicant of the present invention has proposed, in Patent Document 2 (Japanese Patent Application Publication No. 2006-33216), a technique in which the microphone cable is inserted through the flexible pipe in the state in which the shield covered wire having no skin is exposed, and the shield covered wire is brought into contact with the inner surface of the flexible pipe at many points.
According to the invention described in Patent Document 2, the resistance value of the flexible pipe is very low, and the shielding function against electromagnetic waves is improved significantly, whereby the generation of noise can be restrained effectively. However, problems to be solved still remain.
Since the gooseneck condenser microphone is mainly used in a conference room or the like, the length of the support pipe must be increased considering a case where the speaker uses the microphone in a standing-up state. In this case, if the whole of the support pipe is formed by the flexible pipe, it bends excessively, so that the appearance is bad. Therefore, a nonflexible metallic pipe is usually used in a portion in which the length is increased.
However, in the metallic pipe, the shield covered wire of the microphone cable is not always in contact with the inner wall of the metallic pipe. Therefore, for example, when the tilt of the support pipe is changed, loud noise may be generated when the shield covered wire comes into contact with the inner wall of metallic pipe because of looseness of the microphone cable in the metallic pipe (in this specification, this contact is referred to as “looseness contact”).
This noise may also be generated in the case where the microphone cable moves and the shield covered wire comes into contact with the inner wall of the flexible pipe when the flexible pipe is bent, for example, in the state in which the support pipe is made straight, and the shield covered wire of the microphone cable is not in contact with the inner wall of the flexible pipe.
Accordingly, an object of the present invention is to prevent the generation of noise caused by the looseness contact of a shield covered wire with the inner wall of a support pipe by restricting the movement of a microphone cable in the support pipe when the microphone cable is used by being inserted through the support pipe in the state in which the shield covered wire having no skin is exposed throughout the entire length in a gooseneck condenser microphone that supports a condenser microphone unit via the support pipe including a flexible pipe.